The contains a hermetic compressor, 4 heat exchangers, 3 expansion valves and 4 different interchangeable capillaries. The unit can be operated in different modes. 11 manually operated valves allow corresponding changes to be made to the refrigeration circuit. The relevant measured values are displayed directly on indicator instruments. Therefore, sequences and consequences of switching operations within a cooling unit can be directly investigated. Sight glasses in the refrigerant circuit allow direct observation of the refrigerant status. A coaxial tube bundle heat exchanger can be operated as a counterflow condenser or as a parallel flow evaporator. This heat exchanger is used to heat or cool a glycol/water mixture circuit. Refrigerating loads are created by different fan speeds on the finned tube evaporators.